Damage or disease that affects the integral structure of a vertebral body within an individual's spinal column may lead to neurologic impairment with possible permanent damage to the spinal cord as well as improper neck and back alignment. Maintaining anatomic spacing within the spinal column is critical to ensuring continued functionality of the spinal cord and nerve roots and avoidance of long term serious neurological impairment.
Typically, spinal implants that are used as a spacer type of device have a fixed overall length and are implanted without the ability to adjust the degree of expansion or curvature. Recent developments of spinal spacers have resulted in devices that may be lengthened in vivo by rotary motion to match the space presented by the missing vertebral body. In use, these spinal spacers can be expanded until the height of the spacer reaches the height of the disc space in which the spacer is inserted. Problems that have been seen with these types of designs include post-placement migration attributable to the forces applied to the implant during use risking the patient to neurologic injury. The proper sizing of the implant relative to the presented clinical space and the achievement of optimum expansion are important to ensure that the implant fills the space and does not loosen or migrate post implantation. To achieve the required distraction, it is important that the implant insertion and distraction instrument provides the user with optimum handling characteristics. Suitable device access ports for height manipulation are very important, as are a good assortment of endplate angulation possibilities.
Spacers that are lengthened by rotary motion may include gear mechanisms that are engaged by and rotated by a driver tool. For some of these spacers, surgeons rely at least in part on tactile feel to determine when the spacer is fully expanded; i.e. when the ends of the spacer contact the vertebrae on either side of the disc space. In an ideal operation, the gear mechanism rotates with very little resistance until the ends of the implant contact the adjacent vertebrae. Once contact occurs, the device exhibits resistance to further expansion, and that resistance can be sensed by the surgeon as a signal that the implant is fully expanded in the disc space. If the gear mechanism does not rotate properly, due to improper engagement with the driver tool, damaged gear teeth, or other reason, then the surgeon may sense resistance from the implant before the implant is completely expanded, referred to herein as “premature resistance”. Premature resistance can make it difficult or impossible for the surgeon to sense precisely when the implant is fully expanded in contact with the adjacent vertebrae. In some cases, premature resistance from the implant can be mistaken as a signal that that the implant is completely expanded, when in fact it is not.